Engines may be operated using boosting devices, such as turbochargers or superchargers, to increase mass airflow into a combustion chamber. Turbochargers and superchargers compress intake air entering the engine using an intake compressor. Further, one or more intake charging devices may be staged in series or parallel to improve engine boost response.
One example of a multi-staged boosted engine is shown by Stewart et al. in U.S. Pat. No. 7,958,730. Therein, a multistage series turbocharger apparatus contains a low pressure turbocharger and a high pressure turbocharger, coordinated to maintain at least one operating parameter by adjusting turbocharger speeds. The low pressure turbocharger allows for quick acceleration to compensate for the slow acceleration of the high pressure turbocharger, also known as turbo lag. The compressor speed of either turbocharger may be regulated by opening and closing a corresponding wastegate. The wastegate may have active vacuum actuation for better controllability and fuel economy.
However the inventors herein have identified potential issues with such engine systems. As one example, wastegate operation may be limited during engine operation at altitude. Specifically, as the ambient (barometric) pressure drops at higher altitudes, the amount of reserve vacuum available in a pressure canister coupled to the wastegate may decrease. The issue may be exacerbated during uphill travel following downhill travel. During downhill braking, the brake system, which is also a vacuum actuator, may consume a significant amount of the reserve vacuum. As a result, during a subsequent uphill tip-in event, when wastegate closing is requested, there may not be sufficient vacuum available. This can result in the wastegate being partly closed instead of being fully closed, with a loss in transient boosted engine performance.
The inventors herein have recognized that an electric boost provided by an electric motor-driven supercharger compressor can have a substantially immediate impact on boost pressure in a staged engine system. The electric boost can be advantageously used to supplement boost during conditions when the wastegate is deficient. In one example the above issues may be at least partly addressed by a method for a boosted engine, comprising: accelerating each of a first, upstream compressor and a second, downstream compressor to provide a flow of compressed air to a piston engine, the first compressor driven by an electric motor, the second compressor driven by a turbine; and adjusting operation of the first compressor responsive to altitude of vehicle operation. In this way, electric boost may be used for boost control during conditions when wastegate vacuum is limited.
As one example, an electric supercharger including a compressor driven by an electric motor may be staged upstream of a turbocharger including a compressor driven by an exhaust turbine. A vacuum actuated wastegate valve may be coupled across the exhaust turbine. To reduce turbo lag, while the turbocharger compressor spins up, the electric supercharger may be transiently operated to provide boost pressure. In addition, during tip-in events where the wastegate vacuum is insufficient and wastegate performance is limited, such as at higher altitudes, the electric supercharger may be operated to improve boost pressure control. For example, a feedforward and a feedback gain of an electric motor of the electric supercharger may be tuned more aggressively as a function of the ambient pressure (or altitude). This may include operating the electric motor for a longer duration and/or with a higher output at higher altitudes. In addition, a bypass valve coupled across the supercharger may be held more closed for a longer duration to increase airflow through the supercharger compressor. Concurrently, the wastegate may also be operated with a higher gain tuning. The electric supercharger operation may be continued until the altitude decreases or the available reserve vacuum exceeds the amount required for the desired wastegate actuation. Thereafter, the wastegate may be closed to spin the turbine, and electric boost may be reduced or discontinued while turbocharger boost is increased.
In this way, boosted engine performance at higher altitudes can be improved. The technical effect of operating an electric supercharger to provided boosted air to an engine during conditions when a turbocharger wastegate has reduced performance is that the engine can be provided a desired boosted airflow more rapidly. By also closing an electric supercharge bypass valve more aggressively (e.g., to a higher degree of closure and for a longer duration), airflow through the supercharger, and thereby supercharger output, may be increased. As a result, time-to-torque on tip-in events at elevated altitudes can be improved. By tuning the supercharger control loop more aggressively until the vacuum deficiency at the wastegate goes away, boost pressure may be regulated faster and more accurately. Further, the wastegate control loop may be tuned more aggressively. Overall, boost pressure tracking is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.